This, I feel, would be a best guess effort so feel free to add any comments below.
Since in zero-g, hot air cannot rise above the cold air (no convection in freefall), the hot air gets hotter, giving the flame its blue colour. Since there is no up or down, the flame spread in all directions not just rising above cooler air which gives rise to the tall flames here on Earth. The higher temeratures would also burn the fuel more efficiently – meaning no soot.